_content/doc/modules: add retract directive to go.mod reference

Change-Id: Ib2316795409776a9db2dd6293b76d7c0003ba7f4
Reviewed-on: https://go-review.googlesource.com/c/website/+/335690
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Steve Traut <straut@google.com>
diff --git a/_content/doc/modules/gomod-ref.md b/_content/doc/modules/gomod-ref.md
index c029329..d310e8d 100644
--- a/_content/doc/modules/gomod-ref.md
+++ b/_content/doc/modules/gomod-ref.md
@@ -434,3 +434,93 @@
 For more about version numbers, see [Module version numbering](/doc/modules/version-numbers).
+## retract {#retract}
+Indicates that a version or range of versions of the module defined by `go.mod`
+should not be depended upon. A `retract` directive is useful when a version was
+published prematurely or a severe problem was discovered after the version was
+### Syntax {#retract-syntax}
+retract <var>version</var> // <var>rationale</var>
+retract [<var>version-low</var>,<var>version-high</var>] // <var>rationale</var>
+  <dt>version</dt>
+  <dd>A single version to retract.</dd>
+  <dt>version-low</dt>
+  <dd>Lower bound of a range of versions to retract.</dd>
+  <dt>version-high</dt>
+  <dd>
+    Upper bound of a range of versions to retract. Both <var>version-low</var>
+    and <var>version-high</var> are included in the range.
+  </dd>
+  <dt>rationale</dt>
+  <dd>
+    Optional comment explaining the retraction. May be shown in messages to
+    the user.
+  </dd>
+### Example {#retract-example}
+* Retracting a single version
+  ```
+  retract v1.1.0 // Published accidentally.
+  ```
+* Retracting a range of versions
+  ```
+  retract [v1.0.0,v1.0.5] // Build broken on some platforms.
+  ```
+### Notes {#retract-notes}
+Use the `retract` directive to indicate that a previous version of your module
+should not be used. Users will not automatically upgrade to a retracted version
+with `go get`, `go mod tidy`, or other commands. Users will not see a retracted
+version as an available update with `go list -m -u`.
+Retracted versions should remain available so users that already depend on them
+are able to build their packages. Even if a retracted version is deleted from
+the source repository, it may remain available on mirrors such as
+[proxy.golang.org](https://proxy.golang.org). Users that depend on retracted
+versions may be notified when they run `go get` or `go list -m -u` on
+related modules.
+The `go` command discovers retracted versions by reading `retract` directives
+in the `go.mod` file in the latest version of a module. The latest version is,
+in order of precedence:
+1. Its highest release version, if any
+2. Its highest pre-release version, if any
+3. A pseudo-version for the tip of the repository's default branch.
+When you add a retraction, you almost always need to tag a new, higher version
+so the command will see it in the latest version of the module.
+You can publish a version whose sole purpose is to signal retractions. In this
+case, the new version may also retract itself.
+For example, if you accidentally tag `v1.0.0`, you can tag `v1.0.1` with the
+following directives:
+retract v1.0.0 // Published accidentally.
+retract v1.0.1 // Contains retraction only.
+Unfortunately, once a version is published, it cannot be changed. If you later
+tag `v1.0.0` at a different commit, the `go` command may detect a
+mismatched sum in `go.sum` or in the [checksum
+Retracted versions of a module do not normally appear in the output of
+`go list -m -versions`, but you can use the `-retracted` to show them.
+For more, see [`go list -m`](/ref/mod#go-list-m) in the Go Modules Reference.